Historically, when it was necessary to completely remove a disc from between adjacent vertebrae, the conventional procedure is to fuse the adjacent vertebrae together. This “spinal fusion” procedure, which is still in use today, is a widely accepted surgical treatment for symptomatic lumbar degenerative disc disease. However, reported clinical results vary considerably, and complication rates are considered by some to be unacceptably high.
More recently, there have been important developments in the field of disc replacement, namely disc arthoplasty, which involves the insertion of an artificial intervertebral disc implant into the intervertebral space between adjacent vertebrae, and which allows limited universal movement of the adjacent vertebrae with respect to each other. The aim of total disc replacement is to remove pain generation (caused by a bad disc), restore anatomy (disc height), and maintain mobility in the functional spinal unit so that the spine remains in an adapted sagittal balance. Sagittal balance is defined as the equilibrium of the trunk with the legs and pelvis to maintain harmonious sagittal curves. In contrast with fusion techniques, total disc replacement preserves mobility in the motion segment and mimics physiologic conditions.
One such intervertebral implant includes an upper part that can communicate with an adjacent vertebrae, a lower part that can communicate with an adjacent vertebrae, and an insert located between these two parts. An example of this type of implant is disclosed in U.S. Pat. No. 5,314,477 (Marnay).
While this and other known implants represent improvements in the art of artificial intervertebral implants, there exists a continuing need for improvements in this field.